The long-term goal of this project is to understand at a molecular level the mechanism by which cells and tissues develop tolerance to hormones and drugs. An understanding of the molecular basis of tolerance will be particularly valuable in development of protocols for drug use in long-term therapy, where repeated administration of the drug results in decreased efficacy. Our studies will use the hormone-sensitive adenylyl cyclase system as the model for studies on tolerance (also called desensitization). We will study the desensitization process in three separate systems. These are: (1)\the catecholamine-sensitive adenylyl cyclase of the murine lymphoma cell line S49; (2)\the glucagon-sensitive adenylyl cyclase in the canine kidney cell line MDCK; and (3)\the glucagon-sensitive adenylyl cyclase of liver cells. There is now considerable evidence that there are multiple mechanisms by which desensitization occurs. The three different systems chosen show different types of desensitization. We will biochemically analyze each of these systems to localize the changes that occur during desensitization, on the components of the system, and determine the nature of these changes. We will specifically study alterations of the receptor and the GTP-binding regulatory proteins. We will purify these proteins from control and treated cells and use the pure preparation to characterize the nature of the change that occurs during desensitization. Once the changes are characterized, we will attempt to induce such changes in the purified components and test if we can mimic the behavior of the desensitized system. (D)